This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
It has been already stated that most of these medicines stimulate the circulation, and consequently increase the temperature of the surface. Indeed, this effect is often more obvious in health than their influence over the nervous system. The latter is so diffusive that the balance of the functions is little disturbed; and, no one being prominently affected, there is no striking departure from their healthy condition. Yet some influence upon the nervous functions may almost always be observed. A feeling of cheerfulness, a gentle exhilaration of the spirits, greater vividness of the fancy and energy of intellect, a disposition and capacity for increased muscular action, and some excitement of the organic functions, which are more or less under the influence of the nervous centres, and particularly the function of secretion, may generally be noticed. In great excess, some of them produce disordered sensations in the head, as feelings of fulness, vertigo, and headache; but very rarely do any of them, not belonging also to cerebral stimulants, occasion delirium, intoxication, or stupor. But in disease their effects are very obvious; the most violent apparent disorder of the nervous functions yielding sometimes promptly to their influence.
They are for the most part highly diffusible, acting quickly, and soon ceasing to act. They differ, however, much in this respect; and some of them continue to operate for a considerable time.
Most of them are either volatile, or contain a volatile principle, which is often highly odorous, and generally disagreeably so to those unaccustomed to it, though it is often rendered tolerable and even agreeable by habit. Many persons acquire a strong relish for the smell and taste of assafetida and garlic.
It has been thought by some that the nervous stimulants produce their remedial effects exclusively, or nearly so, through the organ of smell. I have never been of that opinion. They will often operate energetically when taken in the form of pill so as to conceal their smell and taste, and not unfrequently will produce the most prompt and powerful effects when given by enema. In some instances, they do probably act by an impression made on the nostrils, which is conveyed through the communicating nerves to the nervous centres; but, in general, there can be no doubt that it is through the alimentary canal that they affect the system. In relation to some of the more odorous, as musk, assafetida, and garlic, the odour which they impart to the exhalation from the lungs and skin, is an incontestable proof of their absorption; and the strong probability is, in reference to all of them, that the active principle enters the circulation, and is carried with the blood to the parts upon which it is to operate. When their volatile principle is inhaled into the lungs, it finds a ready entrance into the circulation; and some of the class, when applied externally, are absorbed with considerable facility; as is certainly the case with garlic, and probably with assafetida.